Endotoxin (the lipopolysaccharide component of Gram-negative bacteria) and the pro-inflamatory cytokines it induces (interleukins, tumor necrosis factor, and interferon gamma) play and important role in inducing nitric oxide production, and participate in the initiation, propagation and extension of endotoxin-induced organ damage in the host. We utilized Electron Paramagnetic Resonance (EPR) in these studies to measure the signal from tissue borne NO. Diethyldithiocarbamate (DETC) was injected simultaneously with ferrous sulfate to from a hydrophobic adduct which then complexed with NO. The resulting complex has a characteristic three-line EPR signal. Endotoxin was injected as a bolus dose, and the signal arising from NO-Fe-(DETC)2 monitored by placing the animal inside a whole body resonator. The data illustrate how the sensitivity of our EPR spectrometer (operating at 1. GHz) enabled us to measure the time course of NO production non-invasively, and directly from the liver during an induced spectic episode in mice. We were able to assess the effects of inhibitors of NOS, and the anti-inflammatory cytokine, Interleukin-13, on tissue NO levels in vivo. Increased levels of NO were observed at early (within 10 min) and late (at 6 hrs) time points following endotoxin injection. IL-13 treatment reduced the NO detectable at 6 hrs by 80%, and implies that this may be valuable as a means of therapy during sepsis.